1. Field of the Invention
The present invention relates to a focus state detection device, more particularly, it relates to a device which is arranged to detect the focussing state of the optical beam of the subject to be photographed through the finder system which is arranged independent from the photographing lens system.
2. Description of the Related Art
There are two ways to detect the focus state of the subject image formed on the film surface. One is the TTL detection method in which a part of the beam flux is guided to a detector such as a CCD after passing through the lens, which method is often used in a single lens reflex camera. The second way is to guide the optical flux which passed a distance measurement system disposed near the finder arranged independent from the photographing lens system, which way is often used in a lens shutter camera.
The focus state detection methods adopt a phase difference detection process as described below.
First, in the distance measuring system, the optical flux from the subject is divided into two portions which are symmetric with each other with respect to the optical axis of the measurement system in the distance measuring device so that two images are formed again for respective portions of the flux. The focus state is detected from the relative position between the two images so that the defocus amount information can be obtained when the image of the subject is dislocated from the focus point and formed before or behind the focus point.
A photoelectric element such as a CCD is used for detecting the relative position of the two images in the above-mentioned focus state detection device. With the use of such a photoelectric element, it becomes possible to detect not only whether the lens is in focus or not but also measure the defocus amount at a high speed by processing the output from the element completely electrically so as to perform calculations on the basis of the output.
With regard to the focus state detection device, Japanese Patent Application Laying Open (KOKAI) No. 62-192732 discloses an electric circuit for obtaining the drive amount of the lens so as to arrange it in focus.
Also, Japanese Patent Application Laying Open (KOKAI) No. 62-125311 discloses a technique for avoiding defocus of the lens when the subject is moving at a high speed.
Further, Japanese Patent Application Laying Open (KOKAI) No. 60-33522 discloses a technique for avoiding the focus error when the luminance of the subject is suddenly changed.
However, in accordance with the focus state detection device according to the prior art, the defocus amount data has to be measured for several times in order to raise the distance measurement accuracy and anticipate the motion of the subject. In this case, every time the distance is measured, the two symmetric areas of the flux are detected by the CCD so as to obtain the phase difference between the two images in respective areas and the driving amount of the photographing lens for focosing the lens is calculated from the data of the phase difference output from the CCD. This results in an increase of time for measuring the distance and detection of the focus state. That is, the total detection time is increased to the time of detection for one detection motion multiplied by the number of the detection times.
Also, in accordance with the detection device mentioned above, the optical finder system and the distance measuring optical system are separately arranged in such a way that the three positions, i.e., the photographing plane in the optical photographing system, the viewing plane in the finder system and the distance measurement portion in the distance measuring optical system are arranged at the same position so as to avoid the parallax that is the dislocation of the photographing plane in the lens system with respect to the viewing plane in the finder according to the distance of the subject to be photographed. To avoid the parallax, the device is so arranged that a distance frame is disposed in the finder system or that the view field frame is shifted according to the distance of the subject.
However, since such an arrangement for avoiding the parallax is not applied between the finder system and the distance measuring system, it is difficult to precisely determine the portion where the distance is measured in the view field. Therefore, this results in that the photograph does not fully meet the intention of the user.
For example, the distance measuring area is usually located at the center portion of the view field plane. The viewing portion of the view field plane is changed according to the distance of the subject to avoid the parallax between the photographing plane, which results in that the parallax is generated between the finder optical system and the distance measuring optical system.
Therefore, even though the subject is positioned at the center of the view field plane and the distance thereof is measured by pressing half of the shutter button, it often occurs that the image is defocussed since the portion where the distance is actually measured is dislocated from the center of the view field plane.
Also, in accordance with the focus state detection device mentioned above, to upgrade the reliability of the focus error signal, the device is arranged in such a manner that a light receiving monitor is disposed near the focus error signal detecting unit so that the optical integration functioned by the unit is stopped when the output from the monitor reaches a predetermined value and the unit outputs the integration result therefrom.
Accordingly, the image in the light receiving portion of the focus error signal detector unit is moved according to the state of focus. Due to this motion of the unit, even when the image in the monitor is not changed during the distance measurement operation, the average output from the light receiving portion of the monitor is changed.
Also, when the subject luminance is abruptly changed during the distance measurement operation, the average output from the monitor changes beyond the actual change of the luminance.
The change of the average output results in that the stop point of the optical integration in the light receiving portion of the focus error signal detection unit is influenced from the distance of the subject.
This means that the reliability of the output from the focus error signal detection unit is lowered.